1. Field of the Invention
The present invention is generally directed to removal of blockage of tubular tissue and specifically directed to the dissolution of intravascular thrombi.
2. Description of the Background Art
It is well known that the formation of thrombi (clots) in blood vessels is a serious medical malady. Thrombi are correlated to the formation of plaque buildup in blood vessels and when blockage occurs, it is more a result of the thrombi than of the plaque buildup (which is usually referred to as atherosclerosis when it is involved in arteries).
All thrombi need not be treated interventionally, but in many instances thrombi do, in fact, become life threatening and require removal or at least reduction in size. A thrombus is primarily comprised of red blood cells and fibrin. There are several treatments which could be adapted for the removal of thrombi in vessels which involve intravascular catheters. Most such intravascular catheters have been designed primarily for plaque removal and contain an element that vibrates at ultrasonic frequencies. Representative of such atherectomy catheters are U.S. Pat. Nos. 5,069,664, 4,920,954, 4,898,575, and 4,808,153. Some involve cutting the plaque off of the wall of the vessel using a cutting blade. Some may be adapted to facilitate removal of a thrombus in a vessel. For example, DonMichael, et al., in U.S. Pat. No. 4,870,953, describes an intravascular catheter having a bulbous head at its distal end which vibrates at ultrasonic frequencies. It is suggested that such a tip might be useful for disintegrating a thrombus. DonMichael, et al., also teaches the discharge of a radiographic contrast medium from the catheter tip to enable visualization of the cleared vessel. A second cooling solution may be circulated through the catheter to the tip to prevent overheating of the bulbous tip. All the foregoing intravenous catheters have their shortcomings. None are particularly adapted for removing thrombi.
The use of laser catheters for treatment of thrombi is not uncommon, and significant damage to vessels during this treatment have been reported. The use of drugs for the primary dissolution of these clots is extremely common and is often considered the primary treatment of choice when a thrombus is present. These drugs are referred to as thrombolytic agents (meaning clot dissolution or decomposition). The most common thrombolytic agents (drugs) that are used presently in the treatment of vascular thrombosis are such agents as urokinase, streptokinase, TPA, leech saliva and other such pharmaceutical clot dissolving agents. Significant problems such as hemorrhagic complications, early rethrombosis, prolonged infusion times, costs, significant failure rates, etc., are persistent problems with the use of these pharmaceutical agents. To overcome the aforesaid problems with drugs, an intravascular spraying catheter may be placed in or near a thrombus and the clot periodically sprayed (or pulsed) with a thrombolytic agent which facilitates clot dissolution. Using intermittent spraying of thrombolytic agents may enable the use of less drug over a shorter time period to effect for thrombolysis when compared to the more classical approach of allowing the drug to drip in or near the clot. But even this approach requires excessive time and drug amount. In addition, the use of pulsatile injections of thrombolytic agents may result in pieces of the clot fracturing off from the main body of the clot and causing an embolism which is a danger faced by interventionalists performing this procedure. It is, therefore, desirable to provide an improved catheter for delivering thrombolytic agents which reduce the time and amount of pharmaceutical agent required for thrombolysis and which reduces the danger of embolism.
Stiles, in U.S. Pat. No. 4,692,139 (incorporated herein by reference), describes a catheter for removing obstructions from biological ducts which transmits ultrasonic vibrations to the obstruction to facilitate lysis. Stiles' catheter has means for administering a lysing agent and simultaneously administering ultrasonic vibrations to obstructing material forward of the catheter tip. The Stiles catheter has a vibrating probe which probe (when the catheter is deployed within a vessel) projects from the tip of the catheter. There is no teaching of any advantages to be gained by either (a) vibrating the catheter (as opposed to a probe housed within a catheter), or (b) using low frequencies (frequencies below 1000 Hz). Further, Stiles teaches the use of vibrational frequencies in the range “of at least 60 KHz.” The vibrational frequency employed to effect lysis is an important issue. It is noted that at the frequencies suggested by Stiles' teaching, the wavelength of ultrasound in the probe is   λ  =            v      f        <          1000      f        <          1000              60        ⁢                  ,                ⁢        000            or λ< 1/60 foot. Thus, in Stiles' catheter there are normally many wavelengths of ultrasound between the ultrasonic source and the probe tip. Wherever the probe tip touches the surrounding aspiration tube walls and/or aspirate, energy will be lost due to heating. Thus, it is difficult or impossible to control the amount of ultrasonic vibratory energy reaching the tip of the probe. Depending on the amount of loss of ultrasonic vibrational energy that occurs along the length of the probe (which, of course, depends on the amount of aspirate in the aspirator tube and the amount of mechanical contact between the probe and the surrounding walls) the energy actually delivered to tissue at the probe tip may either ablate or weld tissue, emulsify an obstruction or be insufficient to have any effect on an obstruction.
Lower frequency vibrations (less than 100 Hz) have wavelengths greater than one foot. The amplitude and, therefore, the energy of the low frequency vibration delivered to the tip of a catheter is much more predictable at the lower frequencies and enable more accurate dosimetry. This is because the vibratory loss to surrounding tissue is due to uniform frictional losses along the length of the elongate member (inserted catheter). Stiles' probe, which vibrates at ultrasonic frequencies as noted above, is housed within an aspiration tube where it may unpredictably be loaded by contact with any aspirate that may be present or the surrounding catheter walls. That is, the undesirable coupling of vibratory energy out of the Stiles' probe is unpredictable. It would be desirable to provide an interventional catheter having a structure wherein the vibrating element contacts the tissue along its entire length.
All of the prior art thrombolysis catheters have specified ultrasonic frequencies (above audible frequencies) when advocating adjunctive vibratory waves to assist thrombolysis. Perhaps this is due to the availability of compact solid state crystals that oscillate or may be driven at these frequencies. Perhaps it is the belief that these frequencies assist in “emulsifying” an obstruction such as a thrombus. Whatever the reason, the present teaching surprisingly shows that the application of low frequency mechanical vibrations facilitate thrombus disintegration. Even more surprisingly, this is true even in the absence of an exogenous lysing agent.